Independence is important to intelligent decision making for two reasons. First, it keeps the mistakes that people make from becoming correlated. Errors in individual judgement won't wreck the group's collective judgement as long as those errors aren't systematically pointing in the same direction. One of the quickest ways to make people's judgements systematically biased is to make them dependent on each other for information. Second, independent individuals are more likely to have a new information rather than the same old data everyone is already familiar with. The smartest groups, then, are made up of people with diverse perspectives who are able to stay independent of each other. Independence doesn't imply rationality or impartiality though. You can be biased and irrational, but as long as you're independent, you won't make the group any dumber.